If you have mastered the art and science of backing up and restoring your system with your chosen backup program that will be the only thing you will need to see you through any difficult times. No need for System Repair Disk, running Start-up repair 3 times, accessing the Recovery console and all that jazz. ( I haven't done it in my life for I rely on my backups that have never failed on me.)
If it is an OEM build, you should have one backup of the other OEM partitions too.
Whenever you do a system backup, immediately restore the system with that back up using the Recovery CD of your backup software and make sure that the backup works. Ideally you should have started doing this from day 1.( Keep an ISO image of your Recovery CD on the external drive.)
Keep one copy of your latest backup in another partition in the internal drive apart from the saved one on the external disk. ( just in case the external HDD fails...)
Keep your Factory Recovery CDs safely as a standby after making and storing the ISO images too on your external drive.
It will be a good practice that you have one external disk exclusively for all these backups, plug-in and plug-out only when needed, and not move it unnecessarily. ( Save it from accidental drops.)